In boiling water nuclear reactors, reactor coolant flows through a series of plenums starting with a lower core plenum, the nuclear core itself and an upper core plenum, each lying in communication with one another. The upper core plenum lies below a shroud head which has a series of standpipes that lead steam/water to a series of separators where the two-phase mixture of steam and water is separated. The separated water flows downwardly in an annulus about the core shroud for recirculation. The separated steam flows upwardly of the reactor through a steam dryer for flow outside of the reactor vessel to drive a turbine, typically for generating power.
In BWR's, this flowing mixture of vapor and liquid must be separated efficiently to provide the dry steam required for steam turbine generators. Typical reactor designs employ primary separators, each of which includes a standpipe connected to the upper core shroud and which standpipe is topped with a helical flow diverter to create a swirl flow into an enlarged separation barrel section. The resultant radial acceleration field causes the higher density liquid to move outward and flow as a film on the separation barrel. Radial pick-off rings are provided at one or more axial positions along the barrel to intercept the liquid film flow and separate it from the interior vapor flow. Discharge passages direct the separated water to a water pool which partially submerges the primary separators.
An example of the foregoing typical primary separator is described and illustrated in U.S. Pat. No. 3,902,876, of common assignee herewith. In that patent, there is illustrated a helical flow diverter in the standpipe and which diverter has a relatively large hub blocking flow in the central region. The diverter also includes diverter vanes which intersect the entire flow field to cause swirl flow over the length of the separation barrel downstream of the hub. Pick-off rings are used to intercept liquid films about the periphery of the separation barrel and to divert the separated liquid to discharge passages. Typically, two, three or more pick-off rings are axially spaced along the length of the separation barrel.
Another separator for this purpose is described and illustrated in U.S. Pat. No. 5,130,082, of common assignee herewith. In that patent, a plurality of axially spaced pick-off rings having curved inlet sections are used in conjunction with a centrally located helical swirler in the separation barrel to afford the separation.
Separation performance criteria have been established to limit the carry-over of liquid in the outlet steam flow (typically to less than 10%) and limit the carry-under of steam in the outlet liquid flow (typically to less than 0.25%) over the range of conditions encountered in normal reactor operation and transients. Separation performance is sensitive to total flow rate, the weight fraction of steam in the inlet flow, and depth of submergence into the surrounding pool of water. Acceptable separation performance has been achieved for a limited range of conditions through careful tuning of the length and angle of the helical device, the sizes, locations and configurations of the pick-off rings and the flow restriction in the first discharge passage.
There are advantages to reducing pressure losses caused by BWR primary separators. This is particularly true for BWR's which utilize natural circulation of the coolant where separator pressure losses limit coolant flows. The primary pressure losses of current separator designs are caused by the inlet flow diverter. See U.S. Pat. No. 3,902,876. That diverter includes significant flow area blockage because of its large central hub. It forces all of the two-phase flow to be mixed and diverted into a steep helical pattern as the flow passes through the swirl inducing vanes. These effects combine to create large pressure losses. Consequently, replacement of the inlet flow diverter with improved swirl devices provides significant reductions in separator pressure losses,
Additionally, reducing the sensitivity of separation performance to variations of inlet parameters and submergence has additional advantages. In current designs, that sensitivity is caused by characteristics of the radial pick-off rings which are adverse to effecting efficient separation. The fixed dimension pick-off openings are difficult to match to wide ranges of liquid film flow rates and thicknesses on the separation barrel wall. Due to the axial momentum of both vapor and liquid phases, either phase is driven into the discharge passage when intercepted by a pick-off ring. Thus, when the liquid film is too thin, vapor flow is intercepted and extracted with the liquid discharge flow, resulting in high carry-under. Conversely, when the liquid film is too thick, excess liquid spills past the outer edge of the pick-off ring and is entrained into the central vapor stream. This leads to high carry-over. This sensitivity to liquid film thickness has been mitigated somewhat by incorporating a local restriction into the primary discharge passage. That restriction tends to cause back-up of liquid, filling the pick-off ring opening even when the nominal film thickness is less than the opening. An additional separation sensitivity results from the variation in submergence depth caused by variations in the surrounding pool over during BWR transients. Submergence variation changes the driving pressure differential and therefore the discharge flow. Thus, the ability to just fill the pick-off ring opening with liquid, to minimize carry-over and carry-under, is adversely impacted by variations in submergence depth. The overall separator performance is further complicated by interactions of these pick-off ring effects among the axial stages. Excess carry-over from the pick-off ring of one stage can cause larger liquid pick-off and discharge at a downstream stage, with resultant larger entrained carry-under outside of the separator.